478 SCIENCE PROGRESS. 



where, and may prove to have a more extended distribution. 

 It is a rhombic mineral with simple crystallographic habit 

 and two conspicuous cleavages, and it has a blue colour 

 with strong pleochroism, which, however, is lost in ordinary 

 thin slices. The specific gravity is 3*08, and the composi- 

 tion is expressed by the formula H 4 Ca Al 2 Si 2 IO , suggest- 

 ing" a comparison with the manganese mineral carpholite. 

 To the new mineral the name lawsonite is given. It is 

 associated in the rock with margarite, epidote, actinolite, 

 glaucophane, and garnet. The glaucophane is peculiar as 

 having an extinction-angle of 13 to 15 , much higher than 

 in the normal mineral and perhaps indicating an admixture 

 of the actinolite-molecule. 



A very instructive memoir by Ransome deals with the 

 geology of Angel Island in San Francisco Bay (34). This 

 island consists mainly of the (probably Cretaceous) San 

 Francisco Sandstone, with intercalated beds of radiolarian 

 chert ; but intruded into these are sills of a rock which is 

 doubtfully placed with fourchite, and also a large dyke of 

 serpentine derived from what has been essentially a diallage- 

 rock. The so-called fourchite is composed mainly of augite, 

 not in crystals but in granules of two sizes, with a matrix 

 consisting of an aggregate of minute prisms of some colour- 

 less mineral. The latter is referred to zoisite, and is 

 certainly secondary, possibly derived from felspar. There 

 are varieties of the rock which contain little plagioclase 

 prisms, and in particular a spheroidal variety in which the 

 felspar occurs in skeletons, delicate needles, and brush-like 

 groupings. The typical fourchite of Williams, it should be 

 remarked, contains no felspar. The chief feature of interest, 

 however, is the metamorphism produced by the intrusive 

 masses in the bedded rocks, which are converted near the 

 junction into glaucophane-schists. One of these, where the 

 sandstone borders the main sill of fourchite, consists chiefly 

 of glaucophane, albite, and biotite in varying proportions. 

 In other specimens quartz is the chief constituent. The 

 radiolarian cherts are changed into a rock composed of 

 quartz with needles of glaucophane, actinolite, etc., and in 

 less altered examples blue glaucophane needles have been 



