146 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the several types of conglomerates have been set forth above. It is 
desirable now to make some comparisons to bring out more clearly 
the distinctions between the different types. For this purpose it will 
be convenient to continue the use of the scheme already employed. 
Matrix. According to Geikie the smaller particles of detritus are 
generally less well rounded than those of greater dimensions (A. 
Geikie, p. 162). This is doubtless true of all water-laid deposits. 
Although the matrices of marine conglomerates are sometimes ferrugin- 
ous they are probably characterized ordinarily by clean sands, cross 
stratified and fairly well assorted. Willis, speaking of beach deposits, 
says, (b, p. 487), “The sand is clean and characterized by marked and 
irregular cross-stratification.” Russell (a, p. 45), referring to the 
incrustation of the grains in certain ferruginous deposits, observes 
that if the debris had been deposited in the ocean and exposed to the 
action of waves and currents, the sands would have been more thor- 
oughly assorted than we now find them, and also that the attrition pro- 
duced by the waves under such circumstances would have scoured 
off the incrustation of ferric oxide. Dutton, too, emphasizes the more 
thoroughly assorted condition of marine sediments as opposed espe- 
cially to fluviatile deposits. As regards the latter Dutton states (p- 
220) that material of all sorts is deposited everywhere, yet with a 
tendency to sorting. Probably the littoral deposits of lakes would 
approach marine deposits in uniformity of size and arrangement 
of particles, but with the absence of tides it is doubtful if these charac- 
teristics would be in general so highly developed. 
Estuarine deposits are seen to consist in the main of mixtures of 
sand and clay not very well assorted but relatively fine. ‘The matrices 
of crush-conglomerates would doubtless present much diversity in the 
size and shape but not in the material of their particles. Probably 
glacial deposits display the greatest variation in the character of the 
finer fragments which constitute their matrices. Fluviatile deposits 
may often approach them in heterogeneity of material and arrange- 
ment and in angularity of individual particles. One minute feature 
of distinction may, however, be noticed. In the case of small fluvia- 
tile fragments, which are only slightly rounded, the attrition will 
probably be equally developed on all corners or edges. In the case 
of similar glacial fragments, as shown by the microscopic study of the 
Dwyka Conglomerate (see, p. 130), one edge or corner of a particle 
may be smoothed or rounded while other corners or edges remain 
sharply angular. While, therefore, there is considerable variation 
in the matrices of the various types of conglomerate and one type 
shades into another, the marine deposits may be regarded as pre- 
