The discovery of blank led to a large population growth in california in the mid 1800s

 GOLD

James Marshall's discovery of gold at Sutter's Mill in California in 1848 led to a "gold rush" in the decade that followed, with 1849 seeing a huge influx of people to California.  (Thus we refer to the '49ers.)  The swift settlement of California added incentive to build a transcontinental railway.  The Pacific Railroad Act of 1862 established the charter for doing that.  The First Transcontinental Railroad was completed in 1869, thus making travel and settlement into California even more accessible.

The discovery of  gold   led to a large population growth in California in the mid 1800s

Explanation:

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mass production/ assembly line production

an expansion is one of two basic business cycle phases; the other is contraction. the transition from expansion to contraction is a peak, and the changeover from contraction to expansion is a trough.