476 SCIENCE PROGRESS. 



of the dark silicates, hornblende and biotite, hypersthene 

 and augite, appearing only locally. Quartz also occurs, and 

 in some cases very large crystals of orthoclase. The por- 

 phyritic elements have continued to grow after the intrusion, 

 and the large orthoclase crystals belong wholly to this stage 

 of the consolidation : the author dissents from the view 

 that phenocrysts are always to be regarded as representing 

 the first or intratelluric period of crystallisation and the 

 ground-mass the second period. In these rocks the ground- 

 mass is of very constant characters, being essentially a 

 granular aggregate of orthoclase and quartz. The several 

 varieties may be grouped generally as porphyrites, usually 

 hornblende-porphyrite, in some cases quartz-porphyrite. 



In conclusion we shall briefly notice some of the petro- 

 graphical memoirs issued as bulletins from the laboratory 

 of the University of California, which, under the direction 

 of Lawson is producing some very useful work in the far 

 West. The first instalment, by the Professor himself (29), 

 deals with the geology of Carmelo Bay, some distance 

 south of San Francisco. The chief interest, from a peno- 

 logical point of view, centres in the eruptive rocks to which 

 Lawson has given the name carmeloite. These exhibit 

 variations of characters within certain limits, the silica per- 

 centage ranging at least from 52 to 60, so that the rocks hold 

 in some respects a position intermediate between basalt and 

 andesite. They have in some cases porphyritic augite and 

 plagioclase, and the ground-mass varies from a holocrystal- 

 line one to one rich in glass. The special feature, however, 

 is the presence of crystals of a mineral for which the name 

 iddingsite is proposed. This substance occurs in crystal 

 forms like those of olivine, with a lamellar structure due to 

 a very perfect cleavage, and with a bronzy lustre on the 

 cleavage faces. Optical examination proves the rhombic 

 symmetry of the mineral, and reveals a brown colour with 

 strong pleochroism. Qualitative chemical tests prove it to 

 be a hydrous non-aluminous silicate of iron, lime, magnesia 

 and soda. The characterisation of carmeloite by reference 

 to this mineral seems to imply that the latter is, as Lawson 

 inclines to believe, an original constituent of the rock, and 



