*4.1. Sample Density Calculation*

The banded-rhyolite sample retrieved for laboratory analysis measures 15 × 8 × 5 cm on three axes perpendicular to one another. Due to irregularities in shape, however, it is not accurate to equate volume with a simple multiplication of the measurements in cubic centimeters (600 cm3). The weight of the sample was found to be 843.5 g. After treatment making the sample water-tight, submergence in water registered a displacement equal to 390 mL. Dividing mass by volume yielded a density of 2.16 for banded rhyolite. Checking the laboratory result for volume against the mathematical result, it was found that the actual volume is only 65% of the latter. All of the boulders in the Almeja CBB are crudely shaped with dimensions similar to a shoe box but with irregularities. Roughly the same adjustment regarding irregular shapes was taken into account when correcting for the estimated boulder weight based on the three-dimensions measured in the field.

The banded rhyolite and hyaloclastite typical of the San Basilio region are seldom found elsewhere on the Baja California peninsula, although smaller but similar rhyolite domes occur on Isla San Luis [14] to the north. The style of Pliocene volcanism at San Basilio and Quaternary volcanism at Isla San Luis are favorably compared with Quaternary rhyolites from islands in the Tyrrhenian Sea off Italy [15,16]. Although the text-book value for the specific gravity of massive rhyolite is commonly given as 2.5, values ranging between 1.6 and 2.8 were calculated for samples by Calanchi et al. (1993), from the Aeolian islands [16]. Our result for the banded rhyolite from Ensenada Almeja falls midway within that range.
