442 E. S. Dana — Petrography of the Sandwich Isla7ids. 



represents well the characteristic types of rocks. It numbers, 

 exclusive of the "pumice" and scoria upwards of seventy speci- 

 mens ; of these about fifty have been subjected to microscopic 

 study. In regard to the geographical distribution of the rocks 

 with reference to their relative age but little can be said. A 

 considerable part (Nos. 90-109) are from the talus within 

 the southern crater of Mokuaweoweo against the neck between 

 it and the central pit. (See the map in vol. xxxvi, plate II). 

 A number of others (78-89) are from the eastern side of the 

 central pit ; and in the case of scattering specimens, the spe- 

 cial source is mentioned more minutely beyond, when interest 

 seems to attach to it. 



In general it may be said that all the specimens in hand from 

 Mauna Loa belong to the same class of basaltic lavas, although 

 they vary widely : in color from dark gray to light gray or dull 

 brick-red ; in structure from compact to highly cellular or 

 vesicular ; from those of uniform grain to those which are 

 prominently porphyritic with chrysolite or feldspar ; and in 

 composition from the very highly chrysolitic kinds to the 

 f eldspathic or augitic forms with little or no chrysolite. Speci- 

 mens of pumice-like scoria are largely represented in the col- 

 lection. 



The specimens may be divided pretty sharply into two 

 groups, besides which there are several other types more or 

 less distinct from these. 



Clinkstone-like bccsalt. — The first of these doubtless includes 

 the rock which former observers have spoken of as resembling 

 phonolite, Macroscopically it has a uniform fine-grained 

 texture, for the most part free from vesicles and apparently 

 compact, though often found on closer examination to be 

 minutely porous. The color varies from a dark bluish gray to 

 light gray, and to dull brick-red or brown, the grayish kinds 

 being the most common. The specific gravity varies from 

 2'82 to 3*00.* Many of these specimens, as taken from the 

 talus between the central and southern craters, are in the form 

 of thin slabs and their resemblance to clinkstone in the hand 

 specimen, though not going beyond external aspect, is suffi- 

 ciently close to explain their having been so named. As regards 

 composition the rocks of this type are most strongly marked 

 by the fact that the chrysolite, which is so common in large 

 grains in the other specimens to be described, is absent or only 

 sparingly present. 



The microscopic characters of this group of finegrained 

 compact rocks are also such as readily to distinguish them from 

 the other forms. In general they consist of augite and plagio- 



* Some of the separate determinations on fragments freed from air by boiling 

 are:3-00, 2-94, 3-00, 2-87, 2-82, 3-00, 2-82. 



