GEOLOGY 367 



hornblende-gneiss, and crystalline limestone, which may possibly 

 be older than the Hecla Hook (Ordovician). The Hecla Hook 

 formation is intruded by a remarkable group of potassic 

 igneous rocks, including a range of types from potash-granite, 

 through augite-biotite-syenite, to shonkinite. A simple linear 

 relation in the mineral proportions suggests that crystallisa- 

 tion-differentiation under gravitational influence has been 

 operative. 



F. W. Clarke and H. S. Washington {Proc. Nat. Acad. Sci. 

 U.S., 8, 1922, pp. 108-1 5) have recently completed the computa- 

 tions of the average chemical composition of the igneous rocks 

 of various countries, of the continents, and of the earth's crust, 

 based on the data assembled in Washington's recent collection 

 of igneous rock analyses. In computing the average composi- 

 tion of a lo-mile thick crust the lithosphere is assumed to be 

 made up of igneous rocks, 95 per cent. ; shale, 4 per cent. ; 

 sandstone, 0*75 per cent. ; and limestone, 0*25 per cent. By 

 assuming that the lo-mile crust is composed entirely of igneous 

 rocks, the only difference produced in the resulting composition 

 affects, on the whole, the second figure of decimals in the 

 percentages. 



T. Crook has adversely criticised Clarke and Washington's 

 method of determining the average chemical composition of 

 the earth's crust {Nature, August 19, 1922, pp. 253-5) on the 

 grounds that no allowance is made for the relative magnitudes 

 of the kinds of rock composing the various averages used. 

 Furthermore, he claims that since the granitic portion of the 

 lithosphere is largely restricted to the continents and is probably 

 underlain at a small depth by a basaltic shell, and that the 

 oceans are also probably underlain by basalt, Clarke and 

 Washington's method seriously under-estimates the basicity 

 of the crust even down to depths of 10 miles. The average 

 igneous rock, too, must be of basic composition, instead of 

 intermediate as often claimed. 



Sedimentary Rocks and 'Structures. — In a paper on the im- 

 pressions made by bubbles, raindrops, and other agencies on 

 the surfaces of strata, W. H. Twenhofel {Bull. Amer. Geol. 

 Soc, 32, 192 1 , pp. 359-72) shows, b3^experiment, that impressions 

 which may be taken for raindrop marks may originate in at 

 least nine different ways, namely : (i) Impressions really made 

 by raindrops ; (2) hail impressions ; (3) drip impressions ; 

 (4) spray and splash impressions ; (5) impressions made by 

 floating bubbles in shallow water which become anchored to 

 the bottom by mud films ; (6) impressions due to bubbles of 

 trapped air rising through mud in flooded areas ; (7) impres- 

 sions due to rising and falling of bubbles in very shallow 

 waters ; (8) impressions due to bubbles arising from the decay 



