What evidence would there be within the sedimentary rocks that the sediments that form mountains (like the Himalayas) came from the ocean, rather than just having been deposited in a basin on land?

Fossil records and carbon contents.

Explanation:

The type of the sedimentary that are present in the Himalayas were the marble, dolomite and silts stone and shale and limestone as per the geologic records the carbon dating of the rocks reveals there ages as per the data collected from the accumulation of the sediments. The upliftment of seas and also the fossil life of the plants and animals that were found during that period.

Its decreasing because of illnesses

Answer light in color; silica-rich;