LAWSON "1 

 l'ALACHE J 



The Berkeley Hills. 



433 



these spots show any definite position with reference to the bound- 

 aries of the area, but the lath-shaped crystals of the surrounding 

 rock are arranged tangentially to the oval space, as might be the 

 case in an ordinary vesicle. In short, these spaces seem to be 

 occupied by portions of the magma in which crystallization was 

 checked suddenly at an early stage of development by a cause 

 which did not affect the mass of the rock. Observations were not 

 sufficiently numerous to show whether the occurrence of amyg- 

 dules with these areas was accidental or otherwise. Their pres- 

 ence may indicate that the disturbing cause which affected crystal- 

 lization was of a gaseous nature, but this is not certain. They may 

 possibly be variolitic in nature. Whatever their cause, it must have 

 been of an extremely local character, its influence not extending 

 further than a fraction of an inch. 



Chemical Characters. — From the above description it is clear 

 that the rocks of this group form a well-defined and closely-related 

 series with the structural and mineralogical composition of olivine- 

 basalt, and dolerite. The chemical composition, so far as known, is 

 in agreement with these conclusions. But two complete analyses 

 of the basalts have been made, one specimen selected being from 

 the flow above the Siestan formation, the other a characteristic 

 basalt from the Campan. Silica determinations were made, how- 

 ever, of other basalts of the series, and the results are pre- 

 sented in the accompanying table. The percentage of silica in 

 these several specimens is very uniform, the extreme range being 

 little more than 3 per cent. The composition as a whole is that of 

 normal basalt. It is interesting to note the close parallelism be- 

 tween these analyses and those of the andesites already given'. For 

 comparison, the average of the first three analyses on page 426 is 

 given in column VI. The differences are such as the mineralogical 

 characters of the rocks would lead us to expect, the more basic 

 feldspars and larger olivine content of the basalt being expressed 

 in the lower percentage of silica, and the marked increase of lime 

 and magnesia. The basalts approach very nearly in composition 

 to the dolerite basalts of Etna as shown by the analysis quoted by 

 Iddings.* 



*Origin of Igneous Rocks, Table V, p. 207. 



